While Skyrim suffers from generally weak questlines, its such a complete game that any RPG has to respect it.
I'll start with the good since there is so much of it. From the perspective of a long time series fan, they made several vast improvements. First up is the enemies. In addition to there being a lot more enemies, they are scaled much better with your level. In previous installments like Oblivion, it seemed like you and the enemies were fighting with toy weapons once you got to a high level. In Skyrim they do a good job of making sure that enemies can both deal out and take damage so that you are not forced to sit in front of a goblin spamming the right trigger until it finally dies. There were also major improvements with regard to your offensive capabilities. For one, having the dual hand system is such an improvement to the overall combat mechanics, allowing you so much more diversity in how you approach a battle. Also, even after 100 hours, I still would never tire of the execution cinematics. There isn't too much change to the weapons, magic, and armor systems, which probably is a good thing.
They completely revamped some of the skill "minigames" such as lockpicking, smithing, and speech (thankfully). Lockpicking was inevitably going to be tedious, but is way less frustrating than trying to pick a master lock in oblivion. Smithing is also a major improvement in that you no longer have to deal with the constant wear and tear of your weapons and armor. The graphics are pretty outstanding, but that goes hand in hand with improved technology, Elder Scrolls games have always been at the forefront of graphics. The music is great as always, series fans like myself will find some familiar scores from Morrowind and Oblivion, as well as some great new additions as well.
Some other changes I think they were wise to make is to allow you to have followers (Nobody likes getting overencumbered), Adding perk trees (allows for a much greater level of customization), and the addition of shouts adds a whole new dimension to combat that is really both satisfying and plain amusing. I may be making this up, but it seemed like quests had more choices and branches than in the past as well. There really are many more great qualities of this game, but if you have ever played or seen it played you know what I am talking about.
There were some aspects of Skyrim I was a little disappointed by. Probably, my biggest complaint is that they took out acrobatics. (That says something) But seriously, there was no substitute for jumping 15 feet from rooftop to rooftop like spiderman. I guess I understand why as they made all of the dungeons have the "loopback" exit, but I still was deeply saddened by this decision. I also, thought for the most part the questlines were pretty weak. The civil war was pretty bland, and I don't think I've seen an Elder Scrolls major questline as weak or short as the Companions one (You didn't even have to deal with Maglir defaulting on a contract one time!). At least, they made up for it with a strong "Mages Guild" (Not calling it anything else ever), a very strong thieves guild, and a solid main questline. I just felt like overall there were less memorable characters and moments in the various quests, but maybe that's just nostalgia. Aside from the questlines, there are plenty of random quests, daedric quests, objectives, and plain random ruins to explore. Really there is no shortage of content.
The game still suffers from some bugs and crashes, but that's to be expected in any game this large. In classic Elder Scrolls fashion, on both of my playthroughs I encountered a bug that prevented me from finishing a questline (Thieves guild and Dawnguard), and my game will freeze from time to time, but since autosaves are now so plentiful, you usually will not lose much progress, and sometimes can even reload autosaves to avoid bugs.
To make an observation, it seems like Bethesda removed some of the complexity and features of the game to make it more accessible to the average gamer. I really can't blame them for doing this, and in most cases it actually works out nicely, but I still have to lament the fact that in Morrowind there were probably twice as many skills as in Skyrim and many more spells (Please bring back levitate)
As far as the additional content goes, hearthfire was surprisingly good. I originally scoffed at the idea of an add on for a house, but then found myself spending two straight hours building and decorating it. I unfortunately didn't make it very far in dawnguard before my game glitched, but crossbows seemed like a cool addition. Dragonborn was incredible though. While the island of Solstheim wasn't very big, they really did a great job filling it with rich characters and the nostalgia factor for a morrowind fan like myself was off the charts. They also added a shout that lets you stop dragon fights, genious!
Anyway, its really hard for me to keep a consistent train of thought going when reviewing a game with the sheer amount of content that Skyrim has. Main takeaways: awesome game, improved on many of the weaknesses of Oblivion. Massive amount of content to keep even the most thorough completionist busy for hours on end (The other day I turned it on at noon, played for a half hour looked up and it was 4). Slight loss of complexity and character/questline development, but overall more quests and variety.
Just realized I never once mentioned dragons. So yeah, in Skyrim there are "boss" fights where you fight dragons. While they are awesome and always satisfying, they do become too frequent late in the game, but then again how can I unlock every should without fighting a dragon every time I fast travel somewhere. Overall, great game.
BTW if I don't see the Lord's Mail in the next game, I will be pisssssed